The present invention relates generally to a measurement of optical performance of an optical element, and more particularly to a calculation of two-dimensional optical performance of a target optical system, such as a projection optical system that transfers a mask pattern onto an object to be exposed.
A projection exposure apparatus has conventionally been used to transfer a mask (or reticle) pattern onto an object to be exposed in manufacturing devices, such as semiconductor devices, image pickup devices, display devices and magnetic heads, in the photolithography process. This exposure apparatus is required to transfer the pattern on the reticle onto the object precisely at a predetermined magnification. For this purpose, it is important to use a projection optical system having excellent imaging performance and sufficiently reduced aberration. In particular, due to the recent demands for finer processing to the semiconductor devices, a transferred pattern is more sensitive to the aberration of the optical system. Therefore, there is a demand to measure the optical performance, such as a wavefront aberration, of the projection optical system with high precision.
One conventional method that actually exposes a mask pattern onto a wafer, and observes and inspects the resist image using a scanning electron microscope (“SEM”) or another means has a problem in a long inspection time due to the exposure and development, difficult SEM operations, and bad inspection reproducibility due to errors caused by resist applications and developments. Accordingly, as a solution for these problems, various measuring devicees have conventionally been proposed, such as a shearing interferometer, such as a lateral shearing interferometer (“LSI”), or a Talbot interferometer that utilizes the shearing interferometry. More recently, a line diffraction interferometry (“LDI”) that has a slit used to form an ideal cylindrical or elliptical wave. See, for example, Japanese Patent Applications, Publication Nos. 2000-146705, and 2000-97666.
The LSI and LDI utilize a one-directional slit or diffraction grating. Therefore, the wavefront aberration of the target optical system obtained by analyzing the measured interference fringe is the wavefront aberration that provides a correct relative relationship in only one direction. Therefore, in order to obtain the wavefront aberration that provides a correct two-dimensional relative relationship, it is necessary, for example, to obtain one-dimensional wavefront aberration information in each of two different, e.g., orthogonal directions and to synthesize them with each other. Yet, no concrete synthesizing method has been proposed. Without correctly calculated two-dimensional wavefront aberration, a correct adjustment for eliminating the aberration is unavailable, or a high-quality exposure cannot be obtained.